Make sure DownloadManager is shut down on the UI thread.
Make sure DownloadManager is shut down on the UI thread.

This fixes a pref-related crash.

TBR=darin

BUG=54128
TEST=memory_test and others; download a file and close the browser while it's downloading; should be no DCHECKs/segfaults

Review URL: http://codereview.chromium.org/3356005

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@58378 0039d316-1c4b-4281-b951-d872f2087c98

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